It is possible to obtain the so-called indicative pressure signal representative of the inner pressure of the combustion chamber of an internal combustion engine by providing a pressure sensor such as a piezo-electric element in a bore formed through a member forming the combustion chamber of the engine such as a cylinder head. A pressure gauge may be otherwise interposed between the cylinder head and the cylinder block of the engine, which functions as the pressure sensor for producing the indicative pressure signal.
It will be seen that the internal pressure in the combustion chamber under operation of the engine changes as indicated by a curve A in FIG. 1. When the ignition system of the engine is triggered at an ignition angle .theta.IG, the air-fuel mixture supplied thereto starts firing with a time delay of .theta.d and, subsequently, the internal pressure rapidly increases up to a maximum pressure peak (referred to as an indicative pressure peak hereinafter) and then decreases.
It is known that a crank angle position of the indicative pressure peak has a certain relationship with the state of the engine at which the maximum output is produced, and the indicative pressure peak giving the maximum engine output has been found, by experiment, to be located between 12 to 13 degrees after the top dead center (referred to as ATDC hereinafter) as shown in the drawings. Therefore, ATDC 12 to 13 degrees may be considered as an ideal crank angle region. It is therefore desirable to determine the ignition timing .theta.IG so that the indicative pressure peak occurs within the ideal crank angle region which is ATDC 12 to 13 degrees.
A feed-back ignition timing control system is disclosed in U.S. Pat. No. 4,481,925 issued Nov. 13, 1984. The feed-back ignition timing control system controls the ignition timing of an internal combustion engine in response to the indicative pressure signal to keep the indicative pressure peak position within an optimum region. In this prior art system, contamination of the indicative pressure signal by high frequency noises is ignored by providing a gating function for taking the indicative peak information only during a predetermined crank angle region or zone. The particular crank angle zone is defined by timing pulses generated by a pulse generator including a toothed wheel and a pickup for producing a timing pulse at each time of the passage of the teeth before it. The teeth are mounted on the periphery of the wheel equidistantly such as 60 degrees.
It has been revealed that such a gating function is still insufficient for avoiding unfavorable operations of the system which should be caused by uncertain states either of the engine such as the so-called knocking state or of the feed-back control system per se. The uncertain state of the feed-back system may occur due to external mechanical and electric noises or troubles in the inner pressure sensor.
Such problems as mentioned above may have been encountered in various feed-back control systems responsive to the indicative pressure signal other than the ignition timing control system, such as a fuel injection control system for a diesel engine. In this fuel injection system, the fuel injection timing is regulated in accordance with the peak position information obtained from the indicative pressure signal. Another feed-back control system based on the indicative pressure signal is an automatic transmission control system which controls its operational mode in response to the indicative pressure signal.